Olefin polymerization, especially ethylene polymerization, can benefit from the addition of longer-chain comonomers, such as 1-butene, 1-hexene, and 1-octene, to produce linear low density polyethylene (LLDPE). LLDPE produced from 1-butene, 1-hexene and 1-octene accounts for a large percentage of the polyethylene resin market. In general, polyethylene plants buy butene, hexene and octene, which are produced in separate plants that typically produce a range of even-numbered alpha olefins from ethylene. It can be expensive to purchase these materials, and they add to the complexity of transport, storage and handling. An attractive alternative is to make the comonomer directly from the ethylene at the site where they will be used, if this can be done cleanly and economically.
The review article “Advances in selective ethylene trimerisation—a critical review” by Dixon et al. (J. Organometallic Chemistry 689 (2004) 3641-3668), herein incorporated by reference in its entirety, describes many different catalysts for trimerization. These catalyst systems contain chromium, and with particular ligands, such as aromatic species (e.g., pyrrolyl) or multidentate heteroatomic species. The chromium catalysts are typically activated by alkylaluminum and/or alkyaluminoxane activators. The article also describes Groups 4 and 5 early transition metals, such as Zr, V, Ta and Ti, and Group 8 late transition metals, such as Ni, for showing some activity in trimerization.
Phillips has developed chromium-based catalysts that are selective towards making 1-hexene from ethylene. The major byproduct appears to be 1-decene. U.S. Publication No. 2004/0228775 and U.S. Pat. No. 6,380,451, describe a standalone process for making 1-hexene.
U.S. Pat. No. 5,382,738 discloses catalyst systems comprising inorganic oxides, modified with a metal alkyl and an unsaturated hydrocarbon, which can be used to support a metal source, such as, for example, chromium, and a pyrrole-containing compound. The resultant catalyst systems can be used to oligomerize and/or trimerize olefins via a slurry process.
U.S. Pat. No. 5,523,507 discloses novel chromium-containing compounds prepared by forming a mixture of the chromium salt, a metal amide, and an ether either supported or unsupported. These novel chromium-containing compounds are activated by non-hydrolyzed alkyl aluminum compound and a Lewis acid.
U.S. Pat. No. 5,451,645 discloses novel chromium-containing compounds prepared by forming a mixture of a chromium salt, a metal amide, and an ether. These novel chromium-containing, or chromium pyrrolide compounds, with a metal alkyl and an unsaturated hydrocarbon, can be used as a co-catalyst system in the presence of an olefin polymerization catalyst system to produce a comonomer in-situ with trimerization.
U.S. Pat. No. 5,543,375 discloses a process to stabilize and/or reactivate an olefin production catalyst system, which comprises contacting an olefin production catalyst system, either before or after use, with an aromatic compound.
European Publication No. 0 668 106 discloses a process which will effectively deactivate, inhibit, and/or “kill” an olefin production catalyst, and halt polymer production in an olefin production process. It further provides for a process which can remove an olefin production catalyst from the product stream, and recover catalyst by-products for recycle, and/or recovery.
International Publication No. WO 99/19280 A1 discloses a process in which olefins are trimerized in the presence of a catalyst system comprising a chromium source, a pyrrole containing compound and a metal alkyl. The process is performed in a reactor and provides for a separator for collection of the desired products.
International Publication Nos. WO 2004/056478 and WO 2004/056479, disclose processes and catalysts to prepare an olefinic stream with more than 30% of 1-octene. The catalysts for this system are those that contain chromium or a chromium salt and a heteroatomic ligand.
Several pyridylamine catalyst complexes have been disclosed for the polymerization or copolymerization of ethylene, propylene, isobutylene, octene, and styrene by Symyx Technologies, Inc. in U.S. Pat. Nos. 6,713,577, 6,750,345, 6,706,829, 6,727,361, and 6,828,397. Pyridylamines were also disclosed in U.S. Pat. Nos. 6,103,657 and 6,320,005, assigned to Union Carbide Chemical and Plastics Technology Corporation, in which zirconium was used as the metal center, and the catalyst complex was used to polymerize alpha-olefins, and in U.S. Pat. No. 5,637,660, assigned to Lyondell Petrochemical Company, which also describes Group 4 complexes of pyridylamine ligands. Robertson et al., Inorg. Chem. 42, pp 6875-6885 (2003), discloses chromium complexes of tris(2-pyridylmethyl)amine for ethylene polymerization.
A need exists for improved processes to more effectively generate alpha-olefin comonomers. More particularly, a need exists for controlling and/or mitigating polymeric fouling in olefin oligomerization reactions. Such fouling reduction would provide benefits including but not limited to reducing/minimizing process down time, more efficiently and/or cost effectively producing desired olefin oligomers, reducing oligomerization reaction byproducts, and/or to reducing/minimizing inefficiencies associated with start-ups and shut-downs, among other reasons.